1. Field of the Invention
According to the invention, the device relates to the measurement of a mass flow, such as that of bulk solids consisting of cereal grain, based on the Coriolis measuring principle. The measuring element is formed by an impeller that is driven at a constant speed. This impeller is acted upon axially by a flow of bulk solids. The bulk solids are fed radially by a channel assembly to the guide vanes, which in turn accelerate them to a constant circumferential velocity. The torque required for this is exactly proportional to the weight-based bulk flow. Reference is made to the German patent DE 33 46 145 A1 and the European patent application EP 0 146 902 A2 with regard to the measuring principle; the disclosures of which are herein incorporated by reference.
2. The Prior Art
The quantity to be measured by this weighing system is the exact torque. A power measurement at the motor, a familiar part of the prior art, is too inaccurate. A torque measurement directly on the impeller's drive shaft is very complex, which is why the motor oscillates suspended and the reaction torque is recorded using a weighing cell, in line with the prior art. This torque measurement is very precise. The problem here, however, is in measuring very low amounts of torque at small flow rates due to the relatively large weight of the motor and the bearing friction that occurs.
In the prior art, this problem is corrected by having the impeller itself oscillate suspended and powered by a type of shock absorber. Depending on the required torque, this results in an angular displacement of the drive shaft relative to the impeller and thereby in the actual weight rate of flow. The timing of this angular shift is measured with each revolution by two proximity switches. The time interval between the rigid drive shaft and the spring-driven impeller corresponds to the required torque and thus to the mass flow rate.
This measuring system is described in the German utility model number 201 15 010.7. The problem involved in this arrangement, however, lies in the necessity of situating the impeller in areas that are subjected to dust and in providing a frictionless seal for it.